A cloudy view of cloudiness

Moving satellites may have caused falling measurements of cloud cover.

Katharine Sanderson

Clouds appear smaller when viewed straight on than when looked at askance.Getty

Satellite evidence that cloud levels are decreasing could just be pie in the sky. The trend might simply be a result of where the satellites are positioned.

Data from the International Satellite Cloud Climatology Project (ISCCP) have shown that cloud levels have decreased by up to 4% over the past 20 years. Clouds increase the Earth's ability to reflect sunlight back into space, cooling the planet. So reduced cloud cover has been linked to global warming.

But Amato Evan at the University of Wisconsin at Madison and his colleagues have taken a closer look at the ISCCP data. Cloud cover decreases abruptly when satellites are moved, the team reports in Geophysical Research Letters1.

As more satellites were launched from the mid 1980s through the 1990s, each satellite could narrow its field of view, looking straight down rather than at an angle. And when observed straight on, clouds appear less cloudy.

Evan claims that change of view explains the large drop in cloudiness over the past 20 years that the ISCCP data suggest. And to infer any global trends from these data is a mistake.

"It's not getting less cloudy on Earth," he says. "It looks like it is [from the ISCCP data] but it's not."

Moving around

William Russow of the City College of New York and head of the ISCCP's global processing centre, admits that moving the satellites probably has an effect.

But, he says that Evan's team has not actually measured the size of that effect, and has overstated its case. "Although there is an effect, one cannot draw such a sweeping conclusion," he says.

Climate researcher Martin Wild at the Institute for Atmospheric and Climate Science in Zurich, Switzerland, uses data like that collected by the ISCCP to study changes in the amount of sunlight at the Earth's surface. Pollution, which reflects sunlight, is thought to mask the warming effects of greenhouse gases.

Clouds are one of the most important influences on global temperatures, Wild says, and tracking their fates is important to judge models of climate change.

If Evan is right, then all the trends that depend on these data sets would be questionable. "It's worrisome — we have a large uncertainty in our knowledge over the past 10 or 20 years," says Wild. "If these data have problems, our knowledge has problems."

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Evan is adamant that his work gives no encouragement to climate-change sceptics. Quite the reverse, he says: "This takes one argument away from the global warming sceptics."

If cloudiness had changed substantially, then there might be an argument that it is clouds, not the greenhouse effect, that is causing climate change. But if it is an artefact in the data collection, the cloudiness has actually stayed the same. "You can't explain global warming from these observations of how cloudy it is," Evan says.